Liquefier assembly for additive manufacturing systems, and methods of use thereof

ABSTRACT

A liquefier assembly for use in an additive manufacturing system, which includes a rigid member having a gap, a liquefier tube operably disposed in the gap, one or more heater assemblies disposed in the gap in contact with the liquefier tube, and configured to heat the liquefier tube in a zone-by-zone manner, preferably one or more thermal resistors disposed in the gap between the rigid member and the heater assemblies, and preferably one or more sensors configured to operably measure pressure within the liquefier tube. The one or more heater assemblies may be operated to provide dynamic heat flow control.

CROSS-REFERENCE TO RELATED APPLICATION(S)

Reference is hereby made to co-filed U.S. patent application Ser. No.______, entitled “Liquefier Assembly With Multiple-Zone Plate HeaterAssembly” (attorney docket no. S697.12-0276); and to co-filed U.S.patent application Ser No. ______, entitled “Additive ManufacturingProcess With Dynamic Heat Flow Control” (attorney docket no.S697.12-0277); and to co-filed U.S. patent application Ser. No. ______,entitled “Additive Manufacturing System And Process With Material FlowFeedback Control” (attorney docket no. S697.12-0278).

BACKGROUND

The present disclosure relates to additive manufacturing systems forprinting or otherwise producing three-dimensional (3D) parts and supportstructures. In particular, the present disclosure relates to print headliquefier assemblies for printing 3D parts and support structures in alayer-by-layer manner using an additive manufacturing technique.

Additive manufacturing systems are used to print or otherwise build 3Dparts from digital representations of the 3D parts (e.g., AMF and STLformat files) using one or more additive manufacturing techniques.Examples of commercially available additive manufacturing techniquesinclude extrusion-based techniques, jetting, selective laser sintering,powder/binder jetting, electron-beam melting, and stereolithographicprocesses. For each of these techniques, the digital representation ofthe 3D part is initially sliced into multiple horizontal layers. Foreach sliced layer, a tool path is then generated, which providesinstructions for the particular additive manufacturing system to printthe given layer.

For example, in an extrusion-based additive manufacturing system, a 3Dpart may be printed from a digital representation of the 3D part in alayer-by-layer manner by extruding a flowable part material. The partmaterial is extruded through an extrusion tip carried by a print head ofthe system, and is deposited as a sequence of roads on a platen inplanar layers. The extruded part material fuses to previously depositedpart material, and solidifies upon a drop in temperature. The positionof the print head relative to the substrate is then incremented, and theprocess is repeated to form a 3D part resembling the digitalrepresentation.

In fabricating 3D parts by depositing layers of a part material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of 3D parts under construction,which are not supported by the part material itself. A support structuremay be built utilizing the same deposition techniques by which the partmaterial is deposited. The host computer generates additional geometryacting as a support structure for the overhanging or free-space segmentsof the 3D part being formed. Support material is then deposited pursuantto the generated geometry during the printing process. The supportmaterial adheres to the part material during fabrication, and isremovable from the completed 3D part when the printing process iscomplete.

SUMMARY

An aspect of the present disclosure is directed to a liquefier assemblyfor use in an additive manufacturing system. The liquefier assemblyincludes a rigid member (e.g., a clam or shell block) derived from oneor more thermally-conductive materials, and having a gap extending alonga longitudinal axis. The liquefier assembly also includes a liquefiertube disposed within the gap, and having an inlet end and an outlet endoffset along the longitudinal axis, and a heater assembly disposed inthe gap and in contact with the liquefier tube, where the heaterassembly is configured to heat the liquefier tube in a zone-by-zonemanner along the longitudinal axis. The liquefier assembly furtherincludes a thermal resistor disposed in the gap between the rigid memberand the heater assembly, where the thermal resistor is configured toconduct a portion of the heat from the heater assembly to the rigidmember, and a heat pipe (or other heat sink device) coupled to the rigidmember to draw the conducted heat away from the rigid member.

Another aspect of the present disclosure is directed to a liquefierassembly for use in an additive manufacturing system, which includes oneor more heater assemblies (e.g., a pair of heater assemblies) configuredto receive a liquefier tube, wherein the pair of heater assemblies arein mating contact with and disposed on opposing sides of the retainedliquefier tube. The liquefier assembly also includes one or more thermalresistors (e.g., a pair of thermal resistors) disposed against the pairof plate heaters, opposite of the retained liquefier tube. The liquefierassembly further includes a rigid member configured to retain thethermal resistor(s), the heater assembly or assemblies, and the retainedliquefier tube under compression, where the rigid member is alsoconfigured to conduct heat from the thermal resistor(s). In someembodiments, the liquefier assemblies also includes the liquefier tube,which is preferably replaceable.

Another aspect of the present disclosure is directed to a method forextruding a material from a liquefier assembly in an additivemanufacturing system. The method includes feeding a filament to aliquefier tube of the liquefier assembly, generating heat with a heaterassembly in contact with the liquefier tube, and conducting a firstportion of the generated heat to the liquefier tube to heat the fedfilament. The method also includes drawing a second portion of thegenerated heat to a thermal resistor in contact with the heaterassembly, opposite of the liquefier tube, and, optionally, operablyconducting the drawn second portion of the generated heat to a heat pipe(or other heat sink device).

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the present disclosure.

The term “operably measure” and like refers to a measurement that may bea direct measurement and/or an indirect measurement. For example,operably measuring pressure within a liquefier tube may be performed bydirectly measuring the pressure within the liquefier tube, and/or may beperformed by indirectly measuring the pressure within the liquefier tubeby directly measuring another effect that is based on the pressurewithin the liquefier tube (e.g., an expansion of the liquefier tube, andthe like).

Directional orientations such as “above”, “below”, “top”, “bottom”, andthe like are made with reference to a layer-printing direction of a 3Dpart. In the embodiments shown below, the layer-printing direction isthe upward direction along the vertical z-axis. In these embodiments,the terms “above”, “below”, “top”, “bottom”, and the like are based onthe vertical z-axis. However, in embodiments in which the layers of 3Dparts are printed along a different axis, such as along a horizontalx-axis or y-axis, the terms “above”, “below”, “top”, “bottom”, and thelike are relative to the given axis.

The term “providing”, such as for “providing a material”, when recitedin the claims, is not intended to require any particular delivery orreceipt of the provided item. Rather, the term “providing” is merelyused to recite items that will be referred to in subsequent elements ofthe claim(s), for purposes of clarity and ease of readability.

Unless otherwise specified, temperatures referred to herein are based onatmospheric pressure (i.e. one atmosphere).

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an additive manufacturing system configured toprint 3D parts and support structures with the use of one or more printhead liquefier assemblies of the present disclosure.

FIG. 2 is an exploded view of a print head retaining a liquefierassembly of the present disclosure.

FIG. 3 is a rear, right perspective view of the liquefier assembly.

FIG. 4 is a rear, left perspective view of the liquefier assembly.

FIG. 5 is an exploded view of the liquefier assembly.

FIG. 6 is a top perspective view of the liquefier assembly, where one ofthe resistor blocks of the liquefier assembly is omitted to ease ofviewing.

FIG. 7 is a rear, right perspective view of a liquefier tube, a resistorblock, and heater assemblies of the liquefier assembly, illustrating amultiple heating zone arrangement for dynamically controlling heatflows.

FIGS. 8A and 8B are schematic illustrations of a filament thicknesscross-section and associated thermal profiles as the filament passesthrough a liquefier tube, depicting dynamic adjustments made to heatingzones in the liquefier tube to accommodate changes in material flowrates.

FIG. 9 is a top perspective view of the liquefier assembly, illustratinga technique for operably measuring pressure within a liquefier tube ofthe liquefier assembly based on liquefier tube expansion and clam blockflexing and compression.

FIG. 10 is a graphical plot of extrudate velocities and encoder signals,illustrating material flow acceleration and deceleration response times.

FIG. 11 is a top view of an alternative liquefier assembly of thepresent disclosure, which includes a cylindrical liquefier tube.

FIG. 12 is a top perspective view of an alternative rigid member from aclam block, which includes multiple, separate c-clip rigid members.

DETAILED DESCRIPTION

The present disclosure is directed to a print head liquefier assemblyfor use in an extrusion-based additive manufacturing system to print 3Dparts and support structures in a layer-by-layer manner using anadditive manufacturing technique. As discussed below, the liquefierassembly is uniquely engineered to improve thermal control over themelting and extrusion of consumable materials. In a first embodiment,the liquefier assembly operates with a multiple-zone heating mechanismand optionally one or more heat sink components that together create apush-pull thermal driver for the liquefier assembly. With this design,the liquefier assembly can generate controllable and precise heat flows,thereby providing fast response times and high flow rates duringprinting operations.

In a second embodiment, the liquefier assembly includes one or moreplate heater assemblies configured to transfer thermal energy to aliquefier tube in a zone-by-zone manner. In this embodiment, each plateheater assembly preferably includes multiple conductor traces thatterminate in heating elements, where each heating element defines aheating zone for the liquefier tube. During operation, electrical powermay be relayed from the additive manufacturing system to the heatingelements via conductor traces of the plate heater assembly in acontrolled and independent manner. In some preferred aspects, theliquefier assembly includes a pair of plate heater assemblies disposedon opposing sides of the liquefier tube, where plate heater assembliesmay have the same number of conductor traces and heating elements tomaintain symmetric heating zones along the length of the liquefier tube.

In a third embodiment, the present disclosure is directed to a methodfor dynamically controlling the heat flow transferred to and from theliquefier tube over multiple heating zones. In particular, thistechnique allows dynamic adjustments to be made to the temperatureprofile along the length of the liquefier tube to accommodate changes inmaterial flow rates, as well as other non-steady state conditions thatoccur during a printing operation (e.g., starting, stopping,accelerating, and decelerating). In particular, the dynamic controlmethod may vary the temperature difference between upstream anddownstream heating zones relative to the material flow rate, such as byinitially over-shooting a target surface temperature of the material,followed by an undershooting of this temperature. This allows highmaterial flow rates to be achieved, while also reducing the risk ofthermally degrading the consumable material.

In a fourth embodiment, the present disclosure is directed to aclosed-loop method for monitoring and controlling material flow withinthe liquefier tube. As discussed below, it has been found that longstandard liquefiers (e.g., 1.5-inches long) can accelerate the flow of apart or support material very quickly (e.g., in about 10 milliseconds orless). However, these liquefiers also exhibit complex flow decelerationsthat include fast decay portions (e.g., less than about 20 milliseconds)and slow decay portions (e.g., greater than about 100 milliseconds).These slow decay portions are an unexpected issue, which is compoundedby the fact that, in some situations, they are not repeatable oversuccessive decelerations. This unpredictability in the flow decelerationreduces the response time control over the material flow, such as whenslowing down into road corners and stopping at the end of roads.

To compensate for the unpredictability in flow decelerations, theliquefier assembly may be configured to operably measure the pressurewithin the liquefier tube. This allows the liquefier assembly to beoperated in a closed-loop manner with one or more process control loopsto provide flow control feedback, at least for the relatively slow timeresponse components. Compensating for the unpredictability in flowdecelerations can also improve response times during printingoperations, such as during cornering and stopping events.

The liquefier assembly of the present disclosure may be used with anysuitable extrusion-based additive manufacturing system. For example,FIG. 1 shows system 10 in use with two consumable assemblies 12, whereeach consumable assembly 12 is an easily loadable, removable, andreplaceable container device that retains a supply of a consumablefilament for printing with system 10. Typically, one of the consumableassemblies 12 contains a part material filament, and the otherconsumable assembly 12 contains a support material filament. However,both consumable assemblies 12 may be identical in structure.

In the shown embodiment, each consumable assembly 12 includes containerportion 14, guide tube 16, and print heads 18, where each print head 18preferably includes a liquefier assembly 20 of the present disclosure.Container portion 14 may retain a spool or coil of a consumablefilament, such as discussed in Mannella et al., U.S. Publication Nos.2013/0161432 and 2013/0161442; and in Batchelder et al., U.S. patentapplication Ser. No. 13/708,145. Guide tube 16 interconnects containerportion 14 and print head 18, where a drive mechanism of print head 18(or of system 10) draws successive segments of the consumable filamentfrom container portion 14, through guide tube 16, to liquefier assembly20 of the print head 18.

In this embodiment, guide tube 16 and print head 18 are subcomponents ofconsumable assembly 12, and may be interchanged to and from system 10with each consumable assembly 12. In alternative embodiments, guide tube16 and/or print head 18 may be components of system 10, rather thansubcomponents of consumable assemblies 12. In these alternativeembodiments, print head 18 (having liquefier assembly 20) may optionallybe retrofitted into existing additive manufacturing systems.

System 10 is an additive manufacturing system for printing 3D parts ormodels and corresponding support structures (e.g., 3D part 22 andsupport structure 24) from the part and support material filaments,respectively, of consumable assemblies 12, using a layer-based, additivemanufacturing technique. Suitable additive manufacturing systems forsystem 10 include extrusion-based systems developed by Stratasys, Inc.,Eden Prairie, Minn. under the trademarks “FDM” and “FUSED DEPOSITIONMODELING”.

As shown, system 10 includes system casing 26, chamber 28, platen 30,platen gantry 32, head carriage 34, and head gantry 36. System casing 26is a structural component of system 10 and may include multiplestructural sub-components such as support frames, housing walls, and thelike. In some embodiments, system casing 26 may include container baysconfigured to receive container portions 14 of consumable assemblies 12.In alternative embodiments, the container bays may be omitted to reducethe overall footprint of system 10. In these embodiments, containerportions 14 may stand adjacent to system casing 26, while providingsufficient ranges of movement for guide tubes 16 and print heads 18.

Chamber 28 is an enclosed environment that contains platen 30 forprinting 3D part 22 and support structure 24. Chamber 28 may be heated(e.g., with circulating heated air) to reduce the rate at which the partand support materials solidify after being extruded and deposited (e.g.,to reduce distortions and curling). In alternative embodiments, chamber28 may be omitted and/or replaced with different types of buildenvironments. For example, 3D part 22 and support structure 24 may bebuilt in a build environment that is open to ambient conditions or maybe enclosed with alternative structures (e.g., flexible curtains).

Platen 30 is a platform on which 3D part 22 and support structure 24 areprinted in a layer-by-layer manner, and is supported by platen gantry32. In some embodiments, platen 30 may engage and support a buildsubstrate, which may be a tray substrate as disclosed in Dunn et al.,U.S. Pat. No. 7,127,309, fabricated from plastic, corrugated cardboard,or other suitable material, and may also include a flexible polymericfilm or liner, painter's tape, polyimide tape (e.g., under the trademarkKAPTON from E.I. du Pont de Nemours and Company, Wilmington, Del.), orother disposable fabrication for adhering deposited material onto theplaten 30 or onto the build substrate. Platen gantry 32 is a gantryassembly configured to move platen 30 along (or substantially along) thevertical z-axis.

Head carriage 34 is a unit configured to receive one or more removableprint heads, such as print heads 18, and is supported by head gantry 36.Examples of suitable devices for head carriage 34, and techniques forretaining print heads 18 in head carriage 34, include those disclosed inSwanson et al., U.S. Pat. No. 8,403,658; and Swanson et al., U.S.Publication No. 2012/0164256. In some preferred embodiments, each printhead 18 is configured to engage with head carriage 34 to securely retainthe print head 18 in a manner that prevents or restricts movement of theprint head 18 relative to head carriage 34 in the x-y build plane, butallows the print head 18 to be controllably moved out of the x-y buildplane (e.g., servoed, toggled, or otherwise switched in a linear orpivoting manner).

As mentioned above, in some embodiments, guide tube 16 and/or print head18 may be components of system 10, rather than subcomponents ofconsumable assemblies 12. In these embodiments, additional examples ofsuitable devices for print heads 18, and the connections between printheads 18, head carriage 34, and head gantry 36 include those disclosedin Crump et al., U.S. Pat. No. 5,503,785; Swanson et al., U.S. Pat. No.6,004,124; LaBossiere, et al., U.S. Pat. Nos. 7,384,255 and 7,604,470;Batchelder et al., U.S. Pat. Nos. 7,896,209 and 7,897,074; and Comb etal., U.S. Pat. No. 8,153,182.

In the shown embodiment, head gantry 36 is a belt-driven gantry assemblyconfigured to move head carriage 34 (and the retained print heads 18) in(or substantially in) a horizontal x-y plane above chamber 28. Examplesof suitable gantry assemblies for head gantry 36 include those disclosedin Comb et al., U.S. Publication No. 2013/0078073, where head gantry 36may also support deformable baffles (not shown) that define a ceilingfor chamber 28.

In an alternative embodiment, platen 30 may be configured to move in thehorizontal x-y plane within chamber 28, and head carriage 34 (and printheads 18) may be configured to move along the z-axis. Other similararrangements may also be used such that one or both of platen 30 andprint heads 18 are moveable relative to each other. Platen 30 and headcarriage 34 (and print heads 18) may also be oriented along differentaxes. For example, platen 30 may be oriented vertically and print heads18 may print 3D part 22 and support structure 24 along the x-axis or they-axis.

System 10 also includes controller assembly 38, which may include one ormore control circuits (e.g., controller 40) and/or one or more hostcomputers (e.g., computer 42) configured to monitor and operate thecomponents of system 10. For example, one or more of the controlfunctions performed by controller assembly 38, such as performing movecompiler functions, can be implemented in hardware, software, firmware,and the like, or a combination thereof; and may include computer-basedhardware, such as data storage devices, processors, memory modules, andthe like, which may be external and/or internal to system 10.

Controller assembly 38 may communicate over communication line 44 withprint heads 18, chamber 28 (e.g., with a heating unit for chamber 28),head carriage 34, motors for platen gantry 32 and head gantry 36, andvarious sensors, calibration devices, display devices, and/or user inputdevices. In some embodiments, controller assembly 38 may alsocommunicate with one or more of platen 30, platen gantry 32, head gantry36, and any other suitable component of system 10. While illustrated asa single signal line, communication line 40 may include one or moreelectrical, optical, and/or wireless signal lines, which may be externaland/or internal to system 10, allowing controller assembly 38 tocommunicate with various components of system 10.

During operation, controller assembly 38 may direct platen gantry 32 tomove platen 30 to a predetermined height within chamber 28. Controllerassembly 38 may then direct head gantry 36 to move head carriage 34 (andthe retained print heads 18) around in the horizontal x-y plane abovechamber 28. Controller assembly 38 may also direct print heads 18 toselectively draw successive segments of the consumable filaments fromcontainer portions 14 and through guide tubes 16, respectively.

FIG. 2 is an exploded view of an example print head 18, which includeshousing 46 (having housing components 46 a and 46 b), drive mechanism48, liquefier assembly 20 of the present disclosure, and heat sink unit50, which are shown in use with filament 52. Examples of suitablecomponents for housing 46 and drive mechanism 48 include those discussedin Batchelder et al., U.S. Pat. Nos. 7,896,209 and 7,897,074; Swanson etal., U.S. Publication No. 2012/0164256, Koop et al., U.S. patentapplication Ser. No. 13/708,116; and Leavitt, U.S. patent applicationSer. No. 13/708,037.

Liquefier assembly 20 is discussed herein as being configured with aribbon filament and ribbon liquefier architecture. The term “ribbonfilament” as used herein refers to a filament having a substantiallyrectangular, arcuate, and/or an elliptical cross-section along itslongitudinal length, which may optionally include one or more surfacetracks for engaging with drive mechanism 48, such as disclosed inBatchelder et al., U.S. Pat. No. 8,236,227. Correspondingly, the term“ribbon liquefier” as used herein refers to a liquefier (e.g., liquefiertube) having a substantially rectangular, arcuate, and/or an ellipticalinner-channel cross-section along its longitudinal length.

For each of the ribbon filament and the ribbon liquefier, thecross-section has a width and a thickness, where a maximum extent of thewidth is at least about 2.5 times greater than a maximum extent of thethickness. For a ribbon filament or ribbon liquefier having asubstantially rectangular or arcuate cross-section, the cross-sectionmay have sharp corners (i.e., 90-degree corners) and/or rounded corners.In aspects in which the ribbon filament has an elliptical cross-section,the elliptical cross-section preferably has an eccentricity of about 0.8or greater, and more preferably of about 0.9 or greater. Examples ofsuitable ribbon filaments and ribbon liquefier architectures forfilament 52 and liquefier assembly 20 include those discussed inBatchelder et al., U.S. Pat. Nos. 8,221,669; 8,236,227; and 8,439,665,the contents of which are incorporated by reference in their entiretiesto the extent that they do not conflict with the present disclosure.

As will be apparent from the discussion herein, the ribbon filament andribbon liquefier architecture provides several advantages for liquefierassembly 20, such as a convenient mechanism for operably measuringliquefier pressures, as well as improved heat flow control, fastresponse times, and fast material flow rates. However, in alternativeembodiments, as discussed below, liquefier assembly 20 may be configuredfor use with liquefiers and filaments having other cross-sectionalgeometries, preferably cylindrical filaments and liquefiers.

Heat sink unit 50 includes heat pipe 50 a and heat sink 50 b, and is acomponent of liquefier assembly 20 that removes heat that is generatedduring operation, as discussed below. In the shown example, heat sink 50b is a pagoda-fin-style heat sink configured to dissipate heat from heatpipe 50 a, and may be located internally or externally to housing 46 ofprint head 18. For instance, the pagoda-fin-style heat sink 50 b mayinclude a plurality of fins (e.g., 5-15 fins) that each have a suitablesurface area (e.g., 1-5 inches in diameter). Heat sink 50 b may alsooperate in conjunction with an active cooling unit, such as a forcedsupply of air blown toward (or drawn away from) heat sink 50 b. Inalternative embodiments, heat sink 50 b may exhibit a variety ofdifferent fin geometries and arrangements.

As further shown, drive mechanism 48 is located upstream from liquefierassembly 20, and is configured to feed successive segments of a filament52 from guide tube 16 to liquefier assembly 20 under motorized power.Drive mechanism 48 also preferably includes a skid plate or othersuitable bearing surface (e.g., a rotating bearing surface, not shown)configured to support the opposing side of filament 52 while drivemechanism 48 is engaged with filament 52. As used herein, the terms“upstream” and “downstream” are made with reference to a filament feeddirection, as illustrated by arrow 53.

Liquefier assembly 20 thermally melts the received successive segmentsof filament 52, where the molten portion of the filament material formsa meniscus around the unmelted portion of filament 52. During anextrusion of the molten material from liquefier assembly 20, thedownward movement of filament 52 functions as a viscosity pump toextrude the molten material as an extrudate for printing 3D part 22 orsupport structure 24. As such, the extrudate flow rate is based in largepart on the pressure within liquefier assembly 20, where the pressure isdue to the viscosity pump action and the thermal expansion of thefilament material upon melting. Accordingly, the highest pressure levelswithin liquefier assembly 20 tend to reside around the meniscus.

Changes in the material flow rate of the extrudate, such as whenstarting, stopping, accelerating, and decelerating, or when faster orslower printing rates are desired, are controlled by changing the feedrate of filament 52 with drive mechanism 48, based on drive commandsfrom controller assembly 38. However, the flow rate of the extrudate outof liquefier assembly 20 does not always immediately respond the same tochanges in the feed rate of filament 52, and exhibits a response timedelay after the change in feed rate. For example, the extrudate may flowat different rates from liquefier assembly 20 for the same instantaneousfeed rate of filament 52 into liquefier assembly 20. This is due tonumerous non-steady state conditions within liquefier assembly 20, suchas changes in the melt flow characteristics of the filament material,previous changes in filament feed rates and extrudate flow rates (e.g.,during previous starting, stopping, accelerating, and/or decelerating),response time delays, and the like.

In an open loop design, without any feedback measurements of theextrudate, controller assembly 38 typically operates print head 18 basedon predictive models on how the extrudate will flow. However, due to thevirtually unlimited 3D part geometries that can be printed with system10, it is difficult to predict how print head 18 will function in everysituation. Furthermore, open loop designs will not detect gradualchanges in print head 18 over time, such as liquefier scaling, materialaccumulation, and the like. Partial tip clogs can alter the performanceof a liquefier without rendering it non-functional.

Instead, as discussed below, liquefier assembly 20 may operate in aclosed-loop manner based on pressure measurements from within liquefierassembly 20. This allows controller assembly 38 to compensate forpressure variations within liquefier assembly 20, thereby moreaccurately controlling the extrudate flow rate out of liquefier assembly20. Additionally, liquefier assembly 20 may operate with temperaturemeasurements along multiple heating zones, which may also be used toprevent temperature fluctuations within liquefier assembly 20, and todynamically control temperature profiles along liquefier assembly 20.These feedback controls can assist in reducing response time delays, andimproving part quality and material flow rates.

Upon exiting liquefier assembly 20, the resulting extrudate may bedeposited onto platen 30 as a series of roads for printing 3D part 22 orsupport structure 24 in a layer-by-layer manner. After the printoperation is complete, the resulting 3D part 22 and support structure 24may be removed from chamber 28, and support structure 24 may be removedfrom 3D part 22. 3D part 22 may then undergo one or more additionalpost-processing steps.

FIGS. 3-6 further illustrate liquefier assembly 20 in use with filament52, where drive mechanism 48 and heat sink 50 a are omitted for ease ofviewing. As shown in FIGS. 3 and 4 (shown at opposite viewpoints fromFIG. 2), liquefier assembly 20 includes clam block 54, a pair of heaterassemblies 56 a and 56 b, liquefier tube 58, a pair of opposing thermalresistors 60 a and 60 b (thermal resistor 60 b is best shown below inFIGS. 5 and 6), tip shield 62, nozzle 64, and sensors 66.

Clam block 54 is an example rigid member that includes a pair ofopposing arms 68 a and 68 b, which extend parallel to each other from abase portion 70. Base portion 70 and arms 68 a and 68 b collectivelydefine a U-shaped gap 72 that extends along a longitudinal axis 73 ofclam block 54 and liquefier tube 58. In the shown arrangement, liquefiertube 58 is disposed within gap 72, between the opposing heaterassemblies 56 a and 56 b. Heater assemblies 56 a and 56 b arerespectively disposed between thermal resistors 60 a and 60 b, wherethermal resistors 60 a and 60 b are respectively held against heaterassemblies 56 a and 56 b by arms 68 a and 68 b. Arms 68 a and 68 bpreferably sandwich liquefier tube 58, heater assemblies 56 a and 56 b,and thermal resistors 60 a and 60 b under sufficient compression tomaintain good interfacial contact between the components, and to preventthem from slipping apart, although it is preferable that liquefier tube58 be replaceable in the assembly.

Clam block 54 may be fabricated from one or more materials that providea strong and rigid structure, and that are preferably thermallyconductive, such as one or more metals (e.g., stainless steel andaluminum). Additionally, clam block 54 is preferably capable ofmaintaining compression on the components retained within gap 72, whilealso being capable of withstanding expansion pressures generated inliquefier tube 58 (when filament 52 is melted and extruded) withoutbreaking or cracking.

Base portion 70 of clam block 54 includes a shaft 74 (best shown belowin FIGS. 5 and 6) that preferably extends through the length of baseportion 70, parallel to gap 72, for receiving heat pipe 50 a. Heat pipe50 a is a sealed thermally-conductive tube configured to draw thethermal energy away from clam block 54 via evaporative cooling. Asdiscussed above, the top end of heat pipe 50 a is connected to heat sink50 b (shown in FIG. 2). The lower portion of heat pipe 50 a may be pressfit into shaft 74 to maintain good interfacial contact with base portion70. In some embodiments, a thermally-conductive hollow sleeve may bedisposed between base portion 70 and heat pipe 50 a, such as forcompliance purposes. In alternative embodiments, heat pipe 50 a may beintegrally formed with base portion 70 such that shaft 74 functions as aportion of heat pipe 50 a.

Heat pipe 50 a also preferably extends along the entire length of baseportion 70, and into tip shield 62. This allows heat pipe 50 a to drawheat from clam block 54 along its entire length, and from tip shield 62.As discussed below, heat pipe 50 a is beneficial for assisting inquickly removing heat from liquefier assembly 20 to rapidly cool downone or more heating zones of liquefier tube 58 when desired.Furthermore, heat pipe 50 a may draw heat away from the inlet end ofliquefier tube 58, above heater assemblies 56 a and 56 b, to preventfilament 52 from softening and buckling at the inlet end.

In the shown embodiment, heater assemblies 56 a and 56 b are a pair ofmirror-image plate heaters configured to transfer thermal energy toliquefier tube 58. As shown in FIG. 3, heater assembly 56 a includes ashim or plate portion 76 a containing multiple conductor traces 78 aseparated by slots that function as thermal dams. Similarly, as shown inFIG. 4, heater assembly 56 b includes a shim or plate portion 76 bcontaining multiple conductor traces 78 b separated by slots thatfunction as thermal dams.

Shim portions 76 a and 76 b are multiple-layer plates that eachpreferably include a rigid base layer (e.g., grade 410 stainless steel)coated with one or more glass and/or other dielectric layers. Suitableplate thicknesses for shim portions 76 a and 76 b range from about 2mils to about 20 mils. In some embodiments, shim portions 76 a and 76 bmay be connected, such as at the opposing side from liquefier tube 58 ina closed book-like manner. This arrangement may assist in retainingliquefier tube 58.

Conductor traces 78 a and 78 b are traces of electrically-conductivematerials routed along the outermost sides of shim portions 76 a and 76b. As discussed below, conductor traces 78 a and 78 b respectivelyterminate in heating elements 82 a (shown below in FIGS. 5 and 7) andheating elements 82 b (shown below in FIG. 6) located on opposing sidesof liquefier tube 58. Additionally, the top-most sections of shimportions 76 a and 76 b may engage with an electrical connector (notshown) of print head 18 and/or head carriage 34, which relays electricalpower from system 10 to heating elements 82 a and 82 b via conductortraces 78 a and 78 b.

In the shown example, heater assemblies 56 a and 56 b each include sevenconductor traces 78 a and 78 b and six heating elements 82 a and 82 b toproduce six heating zones that extend along the length of liquefier tube58. In alternative embodiments, heater assemblies 56 a and 56 b mayinclude different numbers of conductor traces 78 a and 78 b and heatingelements 82 a and 82 b, such as three or more conductor traces 78 a and78 b (and two or more heating elements 82 a and 82 b, for two or moreheating zones), more preferably from three to eleven conductor traces 78a and 78 b (and two to ten heating elements 82 a and 82 b), and evenmore preferably from five to eleven conductor traces 78 a and 78 b (andfour to ten heating elements 82 a and 82 b). As can be appreciated,heater assemblies 56 a and 56 b preferably have the same number ofconductor traces and heating elements (e.g., seven conductor traces andsix heating elements each) to maintain symmetric heating zones along thelength of liquefier tube 58.

In the shown example, liquefier tube 58 is a ribbon liquefier tube witha substantially rectangular cross section for receiving filament 52. Asbriefly mentioned above, in alternative embodiments, liquefier tube 58may have any suitable geometry, preferably a cylindrical tube geometry.Liquefier tube 58 is preferably fabricated from one or more rigid,thermally-conductive materials, such as stainless steel, and may befabricated in a variety of different manners. In a first example, thecross-sectional dimensions of liquefier tube 58 may be attained byflattening or otherwise collapsing or crushing a cylindrical liquefieraround a shim insert.

Alternatively, liquefier tube 58 may be produced by stamping a pair ofmetal sheets into half-sections, which may then be welded or otherwisesealed together to attain the desired cross-sectional dimensions. In afurther example, a U-shaped trench may be laser cut or otherwisemachined into a metal block to form the side walls of liquefier tube 58and nozzle 64, which may then be covered with one or more metalmembranes that encase the trench to form the inner channel.

The substantially-rectangular cross-section of liquefier tube 58 definesopposing faces 58 a and 58 b, which, in the shown embodiment aresubstantially planar faces that are sandwiched between the opposingheater assemblies 56 a and 56 b. This allows heater assemblies 56 a and56 b to conductively transfer thermal energy to liquefier tube 58.

Liquefier tube 58 also has an inlet end 80 a and an outlet end 80 boffset from each other along longitudinal axis 73. Inlet end 80 a isconfigured to receive filament 52 from drive mechanism 48, where inletend 80 a and filament 52 preferably have complementary cross-sectionalgeometries, such as discussed in Batchelder et al., U.S. Pat. Nos.8,221,669 and 8,439,665. Outlet end 80 b is the downstream portion ofliquefier tube 68 and terminates in nozzle 64.

Suitable dimensions for liquefier tube 58 include those discussed inBatchelder et al., U.S. Pat. Nos. 8,221,669 and 8,439,665. In somepreferred embodiments, liquefier tube 58 has a length ranging from about0.3 inch to about 5 inches, more preferably from about 2 inches to about4 inches. Suitable hollow, inner-channel thicknesses between planarfaces 58 a and 58 b of liquefier tube 58 range from about 10 mils toabout 100 mils, and in some embodiments from 30 mils to about 50 mils.Suitable hollow, inner-channel widths between the lateral ends ofliquefier tube 58 (perpendicular to the inner-channel widths) range fromabout 100 mils to about 300 mils, and in some embodiments from 180 milsto about 220 mils. Suitable wall thicknesses for liquefier tube 58 rangefrom about 5 mils to about 20 mils. Suitable dimensions for acylindrical liquefier tube 58 are discussed below.

Thermal resistors 60 a and 60 b are a pair of opposing segmented blocksor assemblies of one or more materials having modest thermalconductivities to draw heat from liquefier tube 58 and heater assemblies56 a and 56 b. As such, while heater assemblies 56 a and 56 b heatliquefier tube 58, thermal resistors 60 a and 60 b draw a portion of thegenerated heat away from heater assemblies 56 a and 56 b, which thenconducts into arms 68 a and 68 b of clam block 54. Because clam block 54is preferably fabricated from one or more thermally-conductivematerials, the drawn heat conducts to base portion 70, which transfersthe heat to heat pipe 50 a. Heat pipe 50 a accordingly draws the heataway from liquefier assembly 20. This creates the push-pull thermaldriver arrangement.

This push-pull thermal driver arrangement is particularly identifiablewhen heater assemblies 56 a and 56 b reduce or discontinue heatingliquefier tube 58 at one or more zones. When this occurs, thermalresistors 60 a and 60 b rapidly draw the residual heat away to quicklycool down liquefier tube 58 at these zones. This provides a high levelof control over the temperature profile along liquefier tube 58, withfast thermal response times and cooling rates.

Suitable materials for thermal resistors 60 a and 60 b include sheetsilicate materials, such as sheet mica. It has been found thatfabricating thermal resistors 60 a and 60 b from materials such as sheetmica provides suitable levels of heat removal, while also providing goodelectrical insulation. Moreover, sheet mica is relatively soft andcompliant, providing good mating interfaces between arms 68 a and 68 bof clam block 54 and heater assemblies 56 a and 56 b.

Additionally, the thermal conductivity of sheet mica in the cleavageplanes of the material is about ten times greater than in directionsperpendicular to the cleavage planes. As such, the sheet mica of thermalresistors 60 a and 60 b is preferably oriented such that the cleavageplains are arranged parallel to the planar faces of heater assemblies 56a and 56 b. This allows thermal resistors 60 a and 60 b to providesufficient levels of thermal resistance, while also allowing suitableamounts of heat to be transferred from heater assemblies 56 a and 56 binto clam block 54.

For example, if the overall thermal resistance between heater assemblies56 a and 56 b and the core region of the filament material in liquefiertube 58 is R, then the fastest thermal response would be obtained forthermal resistors 60 a and 60 b that provide a thermal resistance of Rbetween heater assemblies 56 a and 56 b and clam block 54. However, inmany embodiments, the push-pull thermal driver arrangement of liquefierassembly 20 may operate with a reduced amount of thermal waste.Accordingly, thermal resistors 60 a and 60 b may exhibit thermalresistances between heater assemblies 56 a and 56 b and clam block 54ranging from about R to about 20R, more preferably from about R to about10R, and even more preferably from about R to about 5R.

In an alternative embodiment, thermal resistors 60 a and 60 b may bederived from one or more positive temperature coefficient (PTC)materials, such as barium titanate and/or lead titanate. PTC materialssignificantly increase their electrical resistance over smalltemperature ranges, thereby providing self-regulating temperature zonesalong liquefier tube 58. As such, a single block of a PTC material canemulate an infinite number of temperature zones for liquefier tube 58.In this case, clam block 54 is preferably electrically grounded andheating elements 82 a and 82 b of heater assemblies 56 a and 56 b arepreferably omitted. Instead, the controlled and independent interactionbetween conductor traces 78 a and 78 b and the thermal resistors 60 aand 60 b derived from one or more PTC materials may effectively providean infinite-zone liquefier, where individual zones may be selectivelyshut down.

In an alternative embodiment, liquefier assembly 20 may have one or moreheating zones, each with a fixed operating temperature range as definedby the PTC-material thermal resistors 60 a and 60 b, with only the useof a pair of heater wires. In this case, heater assemblies 56 a and 56 bmay optionally be omitted.

While illustrated with a pair of heater assemblies 56 a and 56 b and apair of thermal resistors 60 a and 60 b, in some embodiments, such asthose in which there is a sufficient amount of thermal conductioncircumferentially around liquefier tube 58, a single heat assembly 56 aand a single thermal resistor 60 a may optionally be used. As such,liquefier assembly 20 may include one or more heater assemblies (e.g.,heater assemblies 56 a and 56 b) and one or more thermal resistors orresistor blocks (e.g., thermal resistors 60 a and 60 b).

At the bottom of liquefier assembly 20, tip shield 62 is connected tothe downstream ends of clam block 54 and heat pipe 50 a. Tip shield 62is thermally isolated from the nozzle 64 and liquefier tube 58 by an airlayer or similar insulator. The surface of the tip shield 62 exposed tothe part under construction should be relatively cooler, to reduce itssurface energy and inclination to attract extrudate.

Nozzle 64 is a small-diameter nozzle of liquefier tube 58 at outlet end80 b, and is configured to extrude molten material at a desired roadwidth. Preferred inner tip diameters for nozzle 64 include diameters upto about 760 micrometers (about 0.030 inches), and more preferably rangefrom about 125 micrometers (about 0.005 inches) to about 510 micrometers(about 0.020 inches). In some embodiments, nozzle 64 may include one ormore recessed grooves to produce roads having different road widths, asdisclosed in Swanson et al., U.S. patent application Ser. No.13/587,002.

As further discussed in Swanson et al., U.S. patent application Ser. No.13/587,002, nozzle 64 may have an axial channel any suitablelength-to-diameter ratio. For example, in some embodiments, nozzle 64may have an axial channel with a length-to-diameter ratio to generatehigh flow resistance, such as a ratio of about 2:1 to about 5:1. Inother embodiments, nozzle 64 may have an axial channel with alength-to-diameter ratio to generate lower flow resistance, such as aratio less than about 1:1. Accordingly, suitable length-to-diameterratios for the axial channel of nozzle 64 may range from about 1:2 toabout 5:1, where in some low-flow resistance embodiments, ratios rangingfrom about 1:2 to about 1:1 may be preferred.

In the shown embodiment, sensors 66 are strain gauges secured to baseportion 70 of clam block 54. As discussed below, sensors 66 areconfigured to measure compressions of base portion 70 due to ballooningexpansions of liquefier tube 58 during operation. This allows thepressure within liquefier tube 58 to be operably measured at one or moreregions along its length. Each sensor 66 preferably communicates with acontrol board of print head 18, a control board of head carriage 34,and/or with controller assembly 38 using one or more electrical,optical, and/or wireless communication lines, to relay pressuremeasurements in a real-time manner. For example, each sensor 66 may beformed on a flexible cable or otherwise attached to contacts on aflexible cable, where the flexible cable may then be operably connectedto the control board of print head 18.

Sensors 66 are illustrated as being offset from each other along thelength of base portion 70, where a first sensor 66 is located at tophalf of base portion 70, and a second sensor 66 is located at a bottomhalf of base portion 70. However, liquefier assembly 20 may include oneor more sensors 66, located at any suitable location along the length ofclaim block 54. For instance, liquefier assembly 20 may include a singlesensor 66 located at or near the downstream end of base portion 70 tooperably measure the pressure within liquefier tube 58 adjacent tooutlet end 80 b and nozzle 64. Additionally, liquefier assembly 20 mayinclude a second sensor 66 located at or near the upstream end of baseportion 70 to operably measure the pressure within liquefier tube 58adjacent to inlet end 80 a. Furthermore, liquefier assembly 20 mayinclude a third sensor 66 located at midpoint location of base portion70 to operably measure the pressure within liquefier tube 58 at itsmidpoint region. An advantage of more than one pressure sensor may bethe ability to infer the axial location of the meniscus region.

FIGS. 5 and 6 further illustrate liquefier assembly 20, where thermalresistor 60 b is omitted in FIG. 6 for ease of viewing. As shown heatingelements 82 a (shown in FIG. 5) are first thin-film heaters disposed onshim portion 76 a between adjacent conductor traces 78 a, and heatingelements 82 b are second thin-film heaters disposed on shim portion 76 bbetween adjacent conductor traces 78 b. Heating elements 82 a and 82 bare the portions of heater assemblies 56 a and 56 b that generate heatin a zone-by-zone manner by the controlled and independent applicationof electrical power through conductor traces 78 a and 78 b.

As further shown in FIGS. 5 and 6, thermal resistors 60 a and 60 brespectively include a set of inner-facing indentations 84 a and 84 bthat align with conductor traces 78 a and 78 b, and a set ofouter-facing indentations 86 a and 86 b. Inner-facing indentations 84 aand 84 b provide clearance for conductor traces 78 a and 78 b, allowingthe compression applied by clam block 54 to seat thermal resistors 60 aand 60 b against heating elements 82 a and 82 b. Similarly, outer-facingindentations 86 a and 86 b reduce thermal spreading along thelongitudinal lengths of thermal resistors 60 a and 60 b (i.e., alonglongitudinal axis 73) to further thermally isolate the heating zonesfrom each other.

In the shown example, thermal resistors 60 a and 60 b are each asegmented block that is separated by inner-facing indentations 84 a and84 b and outer-facing indentations 86 a and 86 b. In particular,outer-facing indentations 86 a and 86 b respectively divide thermalresistors 60 a and 60 b into thermally-isolated segments 87 a and 87 b.However, in alternative embodiments, thermal resistors 60 a and 60 b mayeach be an assembly of distinct segments 87 a and 87 b that arephysically separate from each other.

As shown in FIG. 7, this arrangement of heater assemblies 56 a and 56 band thermal resistors 60 a and 60 b provides six heating zones 88 thatextend along the length of liquefier tube 58. Each heating zone 88extends longitudinally between adjacent conductor traces 78 a, andbetween adjacent conductor traces 78 b, where heating elements 82 a and82 b are in contact with segments 87 a and 87 b of thermal resistors 60a and 60 b. As mentioned above, this thermally isolates each heatingzone 88 from the adjacent heating zones 88, while also allowing thermalresistors 60 a and 60 b to draw heat from the respective heating zones88. In alternative embodiments, conductor traces 78 a and 78 b, andheating elements 82 a and 82 b, may face inwardly against faces 58 a and58 b of liquefier tube 58.

Heater assemblies 56 a and 56 b preferably function as their owntemperature sensors for each heating zone 88 based on the resistance ofheating elements 82 a and 82 b in each heating zone 88. This allowscontroller assembly 38 to control the temperature of each heating zone88 in an independent and closed-loop manner. For example, each heatingzone 88 may be controlled with a half H-bridge driver, which can providea pulse-width modulation drive plus temperature sensing for each heatingzone 88. This high level of temperature control in each heating zone 88reduces the risk of thermally degrading the filament material andheating elements 82 a and 82 b, even at high material flow rates, andallows dynamic heating to be conducted, as discussed below. This reducesthe number of electrical traces required, compared to utilizing separatetemperature sensors, and improves noise immunity, since the temperatureis measured as a change in a large current.

During a printing operation, controller assembly 38 may direct printhead 18 to independently relay electrical power through conductor traces78 a and 78 b to heat up heating elements 82 a and 82 b to individualset point temperatures within each heating zone 88 (or to float todefine adiabatic regions). For example, heating elements 82 a and 82 bmay generate from zero to about 20 watts per heating zone 88 (for atotal of about 120 watts), where the wattage may be independentlycontrolled for each heating zone 88.

The majority of the heat generated from heating elements 82 a and 82 bis transferred to liquefier tube 58 and the filament 52 retainedtherein. This produces a desired temperature profile along liquefiertube 58, which can rapidly heat and melt the material of filament 52 toa molten state for extrusion. Interestingly, it has been found that theheat uptake by the filament material in liquefier tube 58 is non-uniformalong longitudinal length 73. In particular, it has been found that themajority of the heat transferred in an isothermal-wall situation islocated just downstream of the meniscus. Upstream from the meniscus,very little heat is transferred because of the small air gap betweenfilament 52 and the wall of liquefier tube 58. Moreover, downstream ofthe meniscus, the heat transferred drops in an approximately exponentialmanner.

Within liquefier tube 58, the ribbon architecture minimizes the distancethat the heat needs to transfer through filament 52 to reach its coreregion at each heating zone 88 (e.g., compared to a cylindricalfilament). As such, the transferred heat can quickly melt filament 52.For example, based on the zero^(th) order characteristic time to meltribbon filament 52, for a ribbon filament 52 having a 40-mil thickness,and an ABS material composition with a diffusivity of 230 mil²/sec, themelt time is about 0.44 seconds. As such, the first order estimateindicates that the molten ABS material can traverse a 2-inch long heatedliquefier tube 58 in about 0.44 seconds. This corresponds to anextrudate flow rate of about 4,500 micro-cubic inches (mics)/secondwithout thermally degrading the ABS material.

Furthermore, heat pipe 50 a can remove a substantial amount of heat whenliquefier assembly 20 is cooled down. For example, when all heatingzones 88 are operating at about 300° C., the heat drawn through thermalresistors 60 a and 60 b, clam block 54, and heat pipe 50 a can maintainclam block 54 at about 80° C. with a heat removal rate through heat pipe50 a of about 12 watts. Therefore, when the electrical power relayed toheating elements 82 a and 82 b is stopped, heating zones 88 can rapidlycool back down to about 80° C. in a few seconds. This rapid heat removalcan substantially reduce the effects of nozzle oozing that can otherwiseoccur when the cool down rate is slower.

The combination of heat sink unit 50 and clam block 54 also provides aunique design for determining the amount of heat that is transferred tothe filament material in liquefier tube 58. Knowing the amount of heattransfer (e.g., joules) to the filament material can assist incontrolling the amount of extrusion that is generated by the thermalexpansion of the filament material. On the input side, the amount ofheat transferred to the filament material from heater assemblies 56 aand 56 b is readily determined by the electrical power applied to eachheater assembly 82 a and 82 b.

However, to determine the amount of heat transferred to the filamentmaterial, the amount of heat being drawn into clam block 54 also needsto be known. This is achievable when clam block 54 (or other rigidmember) is held at a substantially constant temperature, which itself isachievable with heat sink unit 50. In fact, heat sink unit 50 is capableof holding clam block 54 within about a few degrees of the temperatureof heat sink 50 b, effectively thermally grounding clam block 54. Incomparison, liquefier assemblies that function without heat sink unit50, the temperature of clam block can vary by as much as 70° C. underthe same conditions. As such, heat sink unit 50 also provides a uniquemechanism for determining thermal expansion properties of the filamentmaterial in liquefier tube 58. This accordingly provides greater controlover the extrusion rates of the filament material.

As mentioned above, a second embodiment of the present disclosure isdirected to a method for dynamically controlling the heat flowtransferred to and from liquefier tube 58 over multiple heating zones88. Historically, temperature control of a liquefier tended to hold theentire length of the liquefier at a single set point temperature,typically at a desired exiting temperature. While this reduced the riskof thermal degradation of the consumable material, it also limited thematerial flow rates through the liquefier.

Typically, when a filament is fed into a heated liquefier, the surfaceof the filament begins to heat up and melt, while the core region of thefilament remains relatively cool. This is because the received thermalenergy requires a sufficient amount time to conduct from the surface ofthe filament to melt the core region of the filament. As such, if thefilament is being fed to the liquefier at a rate that is faster thanthis conductance time, the core region of the filament will notsufficiently melt before the given segment of the filament reaches theextrusion nozzle. As can be appreciated, this can result in extrudatewith poor adhesion to the previously deposited material, or potentiallya clogged nozzle.

One conventional technique for increasing material flow rates throughthe liquefier involves setting the temperature control point at the topof the liquefier. However, in some situations, when the filament is fedquickly into the liquefier, the thermocouple tends to chill such thatthe control loop naturally overheats the mid-region of the liquefier.So, while this technique is good for increasing material flow ratesthrough the liquefier, it can also potentially thermally degrade thefilament material.

Liquefier assembly 20, however, having multiple heating zones 88 thatare capable of being independently controlled, allows dynamicadjustments to be made to the thermal profile along the longitudinallength of liquefier tube 58 to account for flow rate changes and othernon-steady state conditions that occur during a printing operation(e.g., starting, stopping, accelerating, and decelerating). This canfurther improve response times and flow rates during printingoperations, while also reducing the risk of thermally degrading the partor support material.

This dynamic heating method involves adjusting the temperatures ofheating zones 88 based on the desired material flow rate throughliquefier tube 58 in an open-loop or closed-loop manner, more preferablyin a closed-loop manner. In particular, heating zones 88 preferably havetemperatures that are similar or substantially the same at low flowrates. This produces a substantially constant temperature profile alongliquefier tube 58. However, as the flow rate increases, the temperaturesbetween heating zones 88 are preferably varied to increase the rate atwhich the filament material melts within liquefier tube 58, while alsoreducing the risk of thermally degrading the material.

For example, FIG. 8A illustrates a low flow rate situation, where theright-side plots 90 a-90 c represent the cross-sectional temperatureprofiles of the material of filament 52 as it passes through liquefiertube 58 in the direction of arrow 53. The cross-section is taken alongthe thickness of filament 52, between faces 58 a and 58 b of liquefiertube 58, where filament 52 has a thickness d (ranging from x=0 to x=d).

In this situation, heating zones 88 may be heated to substantially thesame temperature to generate a substantially constant temperatureprofile 92 along liquefier tube 58. Preferably, as shown in FIG. 8A,each heating zone 88 may be heated to a target temperature T_(m) for thefilament material, such as a suitable melting temperature for extrudingthe material from nozzle 64. For instance, for an ABS filament material,each heating zone 88 may be heated to a target temperature T_(m) about240° C.

Filament 52 is fed to liquefier tube 58 at an initial temperature T_(i)(e.g., ambient temperature), as shown by thermal profile 94 in plot 90a. As filament 52 passes through liquefier tube 58, the surface regionof filament 52 at x=0 and x=d is heated to the target temperature T_(m).In comparison, the core region of the material remains cooler due to thedelayed thermal transfer. This results in a thermal profile 96 shown inplot 90 b. As shown in plot 90 c, the heat eventually travels to thecore region of the material to provide a uniform thermal profile 98 atthe target temperature T_(m). Because the flow rate through liquefiertube 58 is low, the thermal energy has sufficient time to conduct fromthe surface region of the filament material to the core region, therebymelting the entire filament 52 before it reaches nozzle 64.Additionally, the lower temperature (well below the thermal-degradationkinetics threshold (TDKT) of the filament material) reduces the risk ofthermally degrading the filament material, despite the longer residencetime in liquefier tube 58.

However, when an increased material flow rate is desired, thetemperature profile along liquefier tube 58 may be varied to impart heatto filament 52 more quickly than the constant temperature profile 92.This is achievable because, even though the consumable material offilament 52 is heated to an elevated temperature, the exposure durationat the elevated temperature is reduced, allowing the filament materialto remain below its TDKT.

The TDKT is a time-temperature parameter that defines a rate of thermaldegradation of a polymeric material, such as by depolymerization,backbone chain scission, pendant-group stripping, and/or oxidationprocesses. The TDKT reaction rate typically follows the first-orderArrhenius equation, which is substantially linear with time andexponential with temperature. As an example, for a filament materialexposed to a given heating temperature for a given duration, increasingthe exposure temperature by a small amount (e.g., about 10° C.) andreducing the exposure duration by about 50% (i.e., doubling the flowrate) may net about the same thermal reaction rates on the filamentmaterial, although the particular net thermal effects may vary dependingon the filament material composition.

In general, a filament material will remain thermal stable (i.e.,substantially no thermal degradation) as long as it remains below itsTDKT. It is understood that the substantially linear time relationshipof the TDKT is typically cumulative, where multiple exposures to brieftemperature increases, each of which may be below the TDKT of thematerial, will eventually accumulate in duration to exceed the TDKT.However, this typically does not occur in liquefier assembly 20 due tothe consumable nature of filament 52. In other words, the material offilament 52 is typically only exposed to the elevated temperature(s) inliquefier tube 58 a single time before being extruded to produce 3D part22 or support structure 24. Occasionally, the filament material may beheated in liquefier assembly 20 more than once, such as between printingruns and/or between printed layers, where heating zones 88 may be cooleddown and heated up again. Nonetheless, the number of heating cyclesremains low, such that the filament material remains below its TDKT.

In order to substantially increase the flow rate of the filamentmaterial through liquefier tube 58, filament 52 needs to be heated to asignificantly higher temperature than the target temperature T_(m) todrive enough thermal energy into filament 52. For example, when amaterial flow rate of about 10 mics/second is increased to a flow rateof about 10,000 mics/second (i.e., an increase of about three orders ofmagnitude), the increase in temperature can exceed the targettemperature T_(m) by 100° C. or more.

This increased heating, however, presents an issue with respect to theTDKT of the filament material. Once heated to this elevated temperature,the surface region of the filament material typically does not havesufficient time to cool down solely by adiabatic thermal diffusionbefore it begins to thermally degrade (i.e., it will exceed its TDKT).This issue illustrates one of the advantages of the dynamic thermalcontrol attainable with liquefier assembly 20.

As shown in FIG. 8B, the increased heating can be achieved in athermally-stable manner by initially over-shooting the surfacetemperature of filament 52, followed by an undershooting of thistemperature. In particular, one or more of the upstream heating zones 88may be heated to an elevated temperature T_(h) to define heated region100.

For example, for a ribbon filament 52 having a thickness d, an initialambient temperature T_(i), a thermal diffusivity κ, and which is beingfed to liquefier tube 58 in the direction of arrow 53 at a velocityv_(p), the thermal diffusion may be generically described as a systembounded by two parallel planes. As such, the temperature distribution inheated region 100, T_(region)(X, t), may be solved as a relativelysimple case of uniform initial temperature and constant walltemperature, as follows:

$\begin{matrix}{{T_{region}\left( {x,t} \right)} = {T_{h} - {\frac{4}{\pi}\left( {T_{h} - T_{i}} \right){\sum\limits_{n = 0}^{\infty}\; {\frac{1}{{2\; n} + 1}{\sin \left( \frac{\left( {{2\; n} + 1} \right)\pi \; x}{d} \right)}\exp \left\{ \frac{{- {\kappa \left( {{2\; n} + 1} \right)}^{2}}\pi^{2}t}{d^{2}} \right\}}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where T_(h) is the temperature of the heating zones 88 in heated region100.

The thermal power density P_(h)(z) that is transferred to the filamentmaterial accordingly varies with position z along heated region 100, asfollows:

$\begin{matrix}{{P_{h}(z)} = \left. {2\; {k_{p}\left( {\frac{\partial}{\partial x}{T_{region}\left( {x,t} \right)}} \right)}} \right|_{x = 0}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where the position z is the product of time t and the filament velocityv_(p). Combining Equations 1 and 2 provides the following:

$\begin{matrix}{{P_{h}(z)} = {\frac{8}{d}\left( {T_{h} - T_{i}} \right)k_{p}{\sum\limits_{n = 0}^{\infty}\; {\exp \left\{ \frac{{- {\kappa \left( {{2\; n} + 1} \right)}^{2}}\pi^{2}z}{d^{2}v_{p}} \right\}}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

The accumulated thermal energy J_(h)(z) transferred to the filamentmaterial may be represented as follows:

$\begin{matrix}{{J_{h}(z)} = {\int_{0}^{z}{\frac{\partial z^{\prime}}{v_{p}}{P_{h}\left( z^{\prime} \right)}}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

Combining Equations 3 and 4 provides the following:

                                     (Equation  5)${J_{h}(z)} = {\frac{8\; d}{\pi^{2}\kappa}\left( {T_{h} - T_{i}} \right)k_{p}{\sum\limits_{n = 0}^{\infty}\; {\frac{1}{\left( {{2\; n} + 1} \right)^{2}}\left( {1 - {\exp \left\{ \frac{{- {\kappa \left( {{2\; n} + 1} \right)}^{2}}\pi^{2}z}{d^{2}v_{p}} \right\}}} \right)}}}$

The required thermal energy density J_(m) to heat filament 52 enoughsuch that, if the heat is then allowed to diffuse uniformly (e.g., inoutlet region 104), the entire thickness d will be at the targettemperature T_(m), may be represented as follows:

J _(m)=(T _(m) −T _(i)) d Cp μ  (Equation 6)

where here Cp is the heat capacity per mass, and ρ is the mass pervolume. Since κ=k_(p)/C_(p ρ), Equation 6 may be represented as follows:

J _(m)=(T _(m) −T _(i)) d k _(p)/κ  (Equation 7)

Accordingly, the required thermal energy transfers to the filamentmaterial as follows:

                                     (Equation  8)${T_{m} - T_{i}} = {\frac{8}{\pi^{2}}\left( {T_{h} - T_{a}} \right){\sum\limits_{n = 0}^{\infty}\; {\frac{1}{\left( {{2\; n} + 1} \right)^{2}}\left( {1 - {\exp \left\{ \frac{{- {\kappa \left( {{2\; n} + 1} \right)}^{2}}\pi^{2}z_{h}}{d^{2}v_{p}} \right\}}} \right)}}}$

where z_(h) is the length of heated region 100 along longitudinal axis73. Correspondingly, to estimate the required length z_(h) of heatedregion 100, the n=0 term in Equation 8 may be expanded as follows:

$\begin{matrix}{{T_{m} - T_{i}} = {8\left( {T_{h} - T_{i}} \right)\left( \frac{\kappa \; z_{h}}{d^{2}v_{p}} \right)}} & \left( {{Equation}\mspace{14mu} 9} \right) \\{z_{h} = {\frac{T_{m} - T_{i}}{T_{h} - T_{i}}\frac{d^{2}v_{p}}{8\kappa}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

Accordingly, for a ribbon filament 52 having a width w, the maximumvolumetric flow produced by liquefier tube 58 having heated region 100is as follows:

Q_(flow)=w d z_(h)   (Equation 11)

Equations 10 and 11 illustrate the relationship between the elevatedtemperature T_(h) of heated region 100 and the material flow rate. Thiselevated heating in heated region 100 is then followed by a cooledregion 102, in which the electrical currents applied to the associatedheating elements 82 a are substantially reduced. This allows thermalresistors 60 a and 60 b to rapidly draw thermal energy from the surfaceregion of the filament material at these associated heating zones 88.This accordingly prevents the filament material from thermally degradingby exposing filament 52 to the elevated temperature T_(h) for only abrief duration (i.e., the filament material remains below its TDKT atall times).

In comparison, the downstream heating zones 88 may be set to the targettemperature T_(m) to define outlet region 104. These temperatureadjustments between heated region 100, cooled region 102, and outletregion 104 may be conducted by shifting the amount of electrical powerrelayed to heating elements 82 a and 82 b such that the upstreamconductor traces 78 a and 78 b relay greater amounts of electrical powerto the heating elements 82 a and 82 b at heated region 100 compared tothe downstream conductor traces 78 a and 78 b for cooled region 102 andoutlet region 104. In some embodiments, one or more of the heating zones88 in outlet region 104 may float with the surface temperature of thefilament material, such that outlet region 104 functions as an adiabaticregion.

This thermal profiles achieved by heated region 100, cooled region 102,and outlet region 104 are illustrated by the cross-sectional plots106-112 in FIG. 8B. For instance, as shown in plot 106, when filament 52enters liquefier tube 58, the material of filament 52 has asubstantially uniform thermal profile 114 along its thickness at itsinitial temperature T_(i). In other words, plot 106 is the same as plot90 a (shown in FIG. 8A).

However, while passing through heated region 100, the surface regions atx=0 and x=d are heated to the elevated temperature T_(h). In comparison,the core region of the material (i.e., at about x=d/2) remains coolerdue to the delayed thermal transfer. As shown in plot 108, this resultsin a thermal profile 116 as the given segment of the filament materialexits heated region 100 and enters cooled region 102.

As briefly mentioned above, the heating zones 88 in cooled region 102preferably operate at lower temperatures (or are unheated) to allowthermal resistors 60 a and 60 b to rapidly draw excess amounts of thethermal energy from the filament material. This quickly cools down thesurface regions of the filament material edges at x=0 and x=d to ensurethat the filament material remains below its TDKT. However, as shown inplot 110, this thermal transfer also takes time to travel through thefilament material, which generates thermal profile 118 as a thermal waveacross the cross-section of the filament material. In particular, thisthermal wave of profile 118 exhibits temperature peaks at locationsbetween the surface regions (i.e., x=0 and x=d) and the core region(i.e., x=d/2).

As the filament material passes through outlet region 104, the heatretained at the temperature peaks in the thermal wave diffuses uniformly(inwardly and outwardly) to the target temperature T_(m), as illustratedby a substantially uniform thermal profile 120 in plot 112. The use ofheated region 100 and cooled region 102 in this manner allows thefilament material to be rapidly melted, while also only exposing thematerial to the elevated temperature T_(h) for a very brief duration. Inparticular, cooled region 102 cools the surface regions of the filamentmaterial down to reduce the risk of thermally degrading the material.This accordingly launches a thermal wave through the filament materialand allows high material flow rates to be achieved.

Accordingly, when an increase in material flow rate through liquefiertube 58 is desired, the temperature in heated region 100 is increased,and the temperature in cooled region 102 is reduced, where thedifference in temperatures between heated region 100 and cooled region102 may increase with an increase in the material flow rate. Thisaccordingly launches the thermal wave, as discussed above.

On the other hand, when the material flow rate is reduced (i.e., theexposure time is increased), the temperature in heated region 100 ispreferably reduced as well to prevent the filament material fromexceeding its TDKT. The temperature in cooled region 102 may also beincreased as the material flow rate drops to further reduce thetemperature difference between heated region 100 and cooled region 102.This trend may continue until the temperature in each region 100, 102,and 104 is at about the target temperature T_(m). Due to their abilityto dynamically adjust the temperatures of heating zones 88 independentlyof each other in a closed-loop manner, heater assemblies 56 a and 56 bare particularly suitable for generating these types of temperatureprofiles. Additionally, controller assembly 38 may operate heaterassemblies 56 a and 56 to generate a variety of different temperatureprofiles along liquefier tube 58, as particular thermal requirements maydictate.

In comparison, as discussed above, a statically-heated liquefier cantypically extrude consumable materials at relatively low flow rates in athermally-stable manner, or extrude the consumable materials at higherflow rates, but in a less or non-thermally stable manner. However, theseliquefiers are typically not capable of achieving both of theseoutcomes. Liquefier assembly 20, however, can achieve both of theseoutcomes, in addition to generating a variety of other closed-loop,dynamic temperature profiles along liquefier tube 58. As such, liquefierassembly 20 can accommodate a wide variety of material flow rates withgood response times and minimized or otherwise reduced risk of thermaldegradation.

As mentioned above, base portion 70 of clam block 54 may compress due toballooning expansions of liquefier tube 58 during operation, whichallows sensors 66 (e.g., strain gauges) to operably measure the pressurewithin liquefier tube 58. As shown in FIG. 9, during a printingoperation, the heating of liquefier tube 58 melts successive segments offilament 52. The molten portion accordingly forms a meniscus around theunmelted portion of filament 52. As drive mechanism 48 feeds filament 52into inlet end 80 a of liquefier tube 58 (illustrated by arrow 122), thedownward movement of filament 52 functions as a viscosity pump toextrude the molten material from nozzle 64 for printing 3D part 22 orsupport structure 24.

However, the melting of filament 52 causes the consumable material tothermally expand within liquefier tube 58. This, combined with theviscosity pump action of filament 52, pressurizes liquefier tube 58,typically at or below the location of the meniscus. In fact, liquefiertube 58 can be subjected to pressures exceeding about 7 megapascals,which can cause the faces 58 a and 58 b of liquefier tube 58 to balloonoutward depending on where the pressurization is located, as illustratedby arrows 124. This ballooning effect of liquefier tube 58 pressesheater assemblies 56 a and 56 b, and thermal resistors 60 a and 60 boutward in the same directions. This flexes arms 68 a and 68 b of clamblock 54 outward, as illustrated by arrows 126.

As can be seen in FIG. 9, the machining of shaft 74 in base portion 70reduces the solid volume of base portion 70 and provides relatively thinregions 128. These thin regions 128 in turn allow base portion 70 tocompress when arms 68 a and 68 b flex, as illustrated by arrows 130.Sensors 66 may accordingly detect the strain applied to base portion 70when base portion 70 is compressed in this manner.

It is understood that the flexing of arms 68 a and 68 b, and thecompression of base portion 70 are relatively small and do notplastically deform or fracture clam block 54. However, sensors 66 arecapable of detecting very small compression changes in base portion 70,and correlate these compression changes to pressures within liquefiertube 58 to an accuracy of about one pound/square-inch. Because theextrudate flow rate is based in large part on the pressure withinliquefier assembly 20 due to the viscosity pump action, and due tomaterial expansion upon melting, these measured pressures allowliquefier assembly 20 to be operated in a closed-loop manner with one ormore process control loops to provide flow control feedback.

As briefly discussed above, changes in the material flow rate of theextrudate, such as material flow accelerations at start up points,material flow decelerations and accelerations around corners, andmaterial flow decelerations at stopping points, is controlled bychanging the feed rate of filament 52 with drive mechanism 48, based ondrive commands from controller assembly 38. However, the flow rate ofthe extrudate out of nozzle 64 does not always respond the same tochanges in the feed rate of filament 52, and exhibits a response timedelay after the change in feed rate.

It has been found that the response time for material flow accelerationscan be very quick (e.g., about 10 milliseconds or less). As such,liquefier assembly 20 may operate with good response times and flowcontrol during material flow accelerations at start up points andmaterial flow accelerations around corners, for example. This isillustrated by flow plot line 132 and encoder plot line 134 in FIG. 10,where flow plot line 132 is a plot of extrudate velocities exiting aprint head nozzle (e.g., nozzle 64), as measured with velocimetry asdescribed in Batchelder, U.S. patent application Ser. No. 13/840,538.

Encoder plot line 134 is a corresponding plot of a filament drivemechanism encoder, where the filament drive mechanism was switched onand off in a step-wise manner to generate pulses of extrudate flowaccelerations and decelerations. In particular, encoder plot line 134corresponds to feed rates for feeding a filament (e.g., filament 52) toa liquefier, which includes leading edges 134 a corresponding to whenthe filament feed rate is accelerated from zero to a given feed rate,and trailing edges 134 b corresponding to when the filament feed rate isdecelerated from the given feed rate to zero.

As further shown in FIG. 10, as the filament feed rate into theliquefier quickly accelerates (i.e., at leading edge 134 a), theextrudate velocity exiting the liquefier also accelerates very quickly,as illustrated by leading edges 132 a. In particular, leading edges 132a of extrudate plot line 132 closely match the leading edges 134 a ofencoder plot line 134 a.

However, when the filament drive mechanism is stopped and the filamentfeed rate into the liquefier quickly decelerates to zero (at trailingedge 134 b), the extrudate velocity exiting the liquefier exhibitsdelayed and complex flow decelerations, as illustrated by trailing edges132 b. It has been further recognized that these complex flowdecelerations include fast decay portions 136 (e.g., less than 20milliseconds) and slow decay portions 138 (e.g., greater than 100milliseconds).

These slow decay portions 138 are unexpected, and as shown in FIG. 10,do not exhibit a completely repeatable pattern over the successivedecelerations. This unpredictability in the flow decelerationaccordingly reduces the control over the material flow, such as whenslowing down and stopping at the ends of roads.

To compensate for the unpredictability in flow decelerations, thepressure in liquefier tube 58 that is operably measured by sensors 66can be used to predict the extrudate flow rate that exits nozzle 64 in aclosed-loop manner with one or more process control loops. As such,controller assembly 38 preferably operates in a manner that utilizes themeasured pressures in liquefier tube 58 to provide flow control feedbackto predict variations in the extrudate flow rate when liquefier assembly20 is operating in steady state and non-steady state conditions. Forinstance, controller assembly 38 may adjust the feed rate of filament 52(via drive mechanism 48) and/or the temperatures of the heating zones 88along liquefier tube 58 (via heater assemblies 56 a and 56 b) inresponse to the operably measured pressure. This can also improveresponse times during printing operations, allowing print head 18 toproduce 3D part 22 and/or support structure 24 with high partresolutions and fast printing rates.

As mentioned above, in alternative embodiments, liquefier assembly 20may be configured for use with liquefiers and filaments having othercross-sectional geometries. For instance, as shown in FIG. 11, in onepreferred embodiment, liquefier assembly 20 may include cylindricalliquefier tube 58 for use with a cylindrical filament 52. In thisembodiment, because liquefier tube 58 has a cylindrical geometry,liquefier assembly 20 may also include conductive spacers 140 a and 140b having curved inner surfaces configured to mate with liquefier tube58, and planar outer surfaces to mate with the planar heater assemblies56 a and 56 b.

The cylindrical liquefier tube 58 is preferably a thin walled tube,having a wall thickness ranging from about 0.01 inches to about 0.03inches, and more preferably from about 0.015 to about 0.020. Preferredinner diameters for liquefier tube 80 range from about 0.08 inches toabout 0.10 inches, more preferably from about 0.090 inches to about0.095 inches. Additional examples of suitable dimensions for cylindricalliquefier tube 58 include those disclosed in Swanson et al., U.S. Pat.No. 6,004,124; Swanson et al., U.S. Publication No. 2012/0018924; andLeavitt, U.S. patent application Ser. No. 13/708,037.

Conductive spacers 140 a and 140 b are each preferably fabricated fromone or more thermally-conductive materials (e.g., aluminum) to transferheat from heater assemblies 56 a and 56 b to liquefier tube 58.Furthermore, conductive spacers 140 a and 140 b may include indentations142 a and 142 b, which reduce thermal spreading along the longitudinallengths of conductive spacers 140 a and 140 b (i.e., along longitudinalaxis 73) to further thermally isolate heating zones 88 from each other.

In some embodiments, sensors 66 may detect the strain on base portion 70of clam block 54 in the same manner as discussed above. However, incomparison to the ribbon liquefier 58 shown above in FIGS. 2-9, thecylindrical geometry of liquefier 58 shown in FIG. 11 is typically astiffer structure, which reduces the ballooning expansion that isotherwise present with the ribbon liquefier 58. As such, in thisembodiment, sensors 66 are typically required to capable of detectingstrain with higher levels of precision than is otherwise required forthe ribbon liquefier 58.

Alternatively, and more preferably, the pressure within the cylindricalliquefier tube 58 is operably measured using a different technique. Forexample, in embodiments in which filament 52 includes surface tracks forengaging with drive mechanism 48, such as disclosed in Batchelder etal., U.S. Pat. No. 8,236,227, the tractor-engagement between drivemechanism 48 and filament 52 may be used to toggle print head 18 betweena raised and lowered state. In this case, liquefier assembly 20 may alsoinclude one or more force gauge sensors 66 located at outlet end 80 b ofliquefier tube (e.g., at tip shield 62 or nozzle 64), which areconfigured to measure how much force filament 52 is applying toliquefier assembly 20 (minus how hard the extrudate is pressing againstthe 3D part below).

In addition to the feedback control during printing operations, theclosed-loop thermal and/or pressure detections may also be used tocalibrate liquefier assembly 20 between (or during) printing operations.For instance, the temperatures of each heating zone 88 may be monitoredand/or sensors 66 may monitor the pressures within liquefier tube 58while print head 18 extrudes the filament material into a purge bucket,such as disclosed in Turley et al., U.S. Pat. No. 7,744,364. This allowsliquefier assembly 20 to be calibrated to account for manufacturingtolerances, and for subsequent gradual changes over time, such asliquefier scaling, material accumulation, and the like.

FIG. 12 illustrates an alternative to clam block 54. In this embodiment,clam block 54 may be replaced with multiple, separate c-clips rigidmembers 54, each having the same cross-sectional profile as clam block54 (i.e., each c-clip rigid member may include a base portion 70 and apair of arms 68 a and 68 b) to define gap 72. These c-clips rigidmembers 54 may function in the same manner as clam block 54, but mayfurther assist in thermally isolating each heating zone 88, as well asincreasing the thermal draw from liquefier tube 58 (due to the increasedexposed surface area). In fact, in some embodiments where the c-clipsrigid members 54 exhibit sufficient thermal draw to function as astand-alone heat sink, heat sink unit 50 (and optionally shaft 74) maybe omitted.

Furthermore, one or more of the c-clips rigid members 54 may retain asensor 66, if desired. In some embodiments, the base portions 70 of thec-clips rigid members 54 may also be connected with a spine memberextending along longitudinal axis 73. This arrangement allows thec-clips rigid members 54 to maintain their serial relationship alonglongitudinal axis 73, and more readily accommodates the use ofreplaceable liquefier tubes 58.

As can be seen from the above-discussion, liquefier assembly 20 isuniquely engineered to improve thermal control over the melting andextrusion of consumable materials (e.g., filament 52). This is achievedwith the push-pull thermal driver effect from heater assemblies 56 a and56 b, along with resistors blocks 60 a and 60 b, thethermally-conductive clam block 54, and heat sink unit 50. With thisdesign, liquefier assembly 20 can generate controllable and precise heatflows, which provide fast response times and high flow rates duringprinting operations.

Additionally, controller assembly 38 may dynamically control the heatflow transferred to and from liquefier tube 58 over multiple heatingzones 88, such as generating thermal waves along liquefier tube 58,which can increase flow rates during printing operations, while alsoreducing the risk of thermally degrading the consumable material.Furthermore, the pressure within liquefier tube 58 may be operablymeasured, allowing controller assembly 38 to adjust the feeding offilament 52 to liquefier tube 38 in response to the measured pressure tocontrol the material flow rate of the extrudate in a closed-loop manner.This is particularly suitable for compensating for the unpredictabilityin flow decelerations, such as for slow decay portions having responsetimes greater than about 100 milliseconds, which can response timesduring printing operations.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

1-20. (canceled)
 21. A liquefier assembly for use in an additivemanufacturing system, the liquefier assembly comprising: athermally-conductive main body having a first end and a second end and achannel extending from the first end to the second end; a liquefier tubedisposed within the channel, and having an inlet end and an outlet end;a heater assembly disposed in the gap and in contact with the liquefiertube, the heater assembly comprising a plurality of independentlycontrollable heating zones wherein the heater assembly and configured toimpart heat into the liquefier tube and the main body; and a heat sinkunit coupled to the main body and configured to draw heat away from themain member.
 22. The liquefier assembly of claim 21 and furthercomprising a thermal resistor disposed in the channel between theliquefier tube and the main body.
 23. The liquefier assembly of claim22, wherein the thermal resistor comprises a plurality of segmentsconfigured to reduce thermal spreading along the resistor block indirections along the longitudinal axis.
 24. The liquefier assembly ofclaim 21, wherein the main body comprises a base portion and a pair ofspaced apart arm that define the channel.
 25. The liquefier assembly ofclaim 24, wherein the liquefier tube is removably retained within themain body with a compressive force caused by flexing of the spaced apartarms.
 26. The liquefier assembly of claim 21, wherein a portion of theheat sink unit extends from the first end to the second end of the mainbody.
 27. The liquefier assembly of claim 21, wherein the liquefier tubecomprises a ribbon liquefier tube.
 28. The liquefier assembly of claim21, wherein the liquefier tube comprises a cylindrical liquefier tube.29. The liquefier assembly of claim 21, wherein the heat sink unitcomprises a heat pipe configured to utilize evaporative cooling to drawheat away from the main member.
 31. A liquefier assembly for use in anadditive manufacturing system, the liquefier assembly comprising: aliquefier tube; at least one heater assembly having a first side and asecond side, wherein the first side contacts the retained liquefiertube; at least one thermal resistors in contact with the second side ofthe at least one heater assembly; and a main body configured to retainthe at least one thermal resistor, the at least one heater assembly, andthe liquefier tube under compression, wherein the rigid member is alsoconfigured to conduct heat from the at least one thermal resistor. 32.The liquefier assembly of claim 31, where the at least one heaterassembly comprises a pair of heater assemblies having first sides andsecond sides, wherein the first sides of the pair of heater assembliesare positioned on opposite sides of the liquefier tube.
 33. Theliquefier assembly of claim 32, wherein the at least one thermalresister comprises a pair of thermal resistors positioned on the secondsides of the pair of heater assemblies.
 34. The liquefier assembly ofclaim 31, wherein the at least one heater assembly comprises comprisinga plurality of independently controllable heating zones.
 35. Theliquefier assembly of claim 31, and further comprising a heat sink unitconfigured to draw the heat from the rigid member.
 36. The liquefierassembly of claim 35, wherein the heat sink unit comprises a heat pipeconfigured to utilize evaporative cooling to draw heat away from themain member.
 37. The liquefier assembly of claim 31, wherein the atleast one thermal resistor comprises a plurality of segments configuredto reduce thermal spreading along the thermal resistor.
 38. Theliquefier assembly of claim 31, wherein the thermal resistors eachcompositionally comprise a material selected from the group consistingof sheet mica and one or more positive temperature coefficientmaterials.
 39. The liquefier assembly of claim 31, wherein the liquefiertube comprises a ribbon liquefier tube.
 40. The liquefier assembly ofclaim 31, and further comprising a sensor configured to operably measurepressure within the retained liquefier tube.